1. Field of the Invention
The present invention relates to diaphragm devices, such as those used in a vacuum advance mechanism of a distributor for an internal combustion engine, and more particularly to improvements in the mounting and sealing structure of diaphragms therein.
2. Description of the Prior Art
Diaphragm devices are used, for example, in a vacuum advance mechanism of a distributor for an internal combustion engine. In such devices, diaphragms are usually tightly clamped between two rigid flat members at the marginal portions thereof, to define with a housing member a hermetically sealed chamber.
FIG. 1a shows an axial cross section of an example of a diaphragm device which forms part of a vacuum advance mechanism of a distributor for an internal combustion engine. The housing of the device consists of two cup-shaped members, housing top 1a and bottom 1b, and a diaphragm 2 is tightly held between the housing top 1a and the housing bottom 1b at the outer marginal portion thereof, to define with the housing top 1a a vacuum chamber 3a thereabove, and an atmospheric pressure chamber 3b therebelow with the housing bottom 1b. The central portion of the diaphragm 2 is held between a pair of stiffening disks 4a and 4b of metal which are clamped together by rivets 5a and 5b. A control rod 6 is coupled to a central convex portion of the lower stiffening disk 4b at the top end thereof, and extends through a central aperture 1c of the housing bottom 1b. The other end of the control rod 6 is coupled to the ignition advancing plate (not shown) within the distributor to transmit axial displacement of the diaphragm 2 thereto so that the ignition timing is advanced in proportion to the displacement. A helical spring 7 reacting against the bottom portion of the cup-shaped housing top 1a bears against the upper stiffening disk 4a to urge the diaphragm 2 to the neutral or zero advance position. A suction pipe 1d communicates the vacuum chamber 3a to a portion of an intake manifold (not shown) of the internal combustion engine immediately upstream of an unillustrated throttle thereof, so that suction from the chamber 3a through the pipe 1d increases as the degree of opening of the throttle increases. The atmospheric pressure chamber 3b, on the other hand, communicates with the atmosphere through the central aperture 1c formed in the housing bottom 1b. Thus, when the suction within the vacuum chamber 3a increases, the diaphragm 2 is moved upward in the FIG. 1a against the biasing force of the spring 7, due to the pressure difference across the two chambers 3a and 3b. The resulting displacement of the diaphragm 2 is transmitted to the ignition advancing plate within the distributor through the control rod 6 so that the ignition timing is advanced.
FIG. 1b is an enlarged view of the portion within circle A in FIG. 1a, showing the mounting and sealing structure of the outer marginal portion of the diaphragm. The marginal portion of the housing top 1a is formed into a radially extending flat annular flange 10a. The marginal portion of the bottom 1b of the housing is formed into a clamping structure of inwardly open U-shaped cross section, wherein the outer marginal portion 2a of the diaphragm 2 is inserted between a radially outwardly extending flat annular flange portion 10b of the U-shaped marginal structure of the housing bottom 1b and a flange 10a of the housing top 1a, the flange 10a and the marginal portion 2a of the diaphragm 2 being held between the flange portion 10b and the inwardly bent extension 11b of the U-shaped marginal portion of the housing bottom 1b. The two radially extending flat annular portions 10b and 11b of the housing bottom 1b are clamped together by a press, so that the marginal portion 2a of the diaphragm 2 is held tightly between the flanges 10a and 10b of the housing top 1a and the housing bottom 1b, wherein the thickness of the clamped marginal portion 2a of the diaphragm 2 is reduced by the clamping pressure, compared with the original thickness of the non-compressed portion of the diaphragm 2, thereby ensuring airtight sealing thereat.
The mounting structure of the diaphragm shown in FIG. 1b has the following disadvantage. Namely, even when the pressure and the stroke of the clamping press are maintained strictly at predetermined magnitudes, the thickness reduction in the marginal portion 2a of the diaphragm 2 due to the clamping force tends to vary from one device to another and can hardly be maintained at a constant magnitude, due to random variations in the component parts, etc. Thus, if the thickness reduction of the diaphragm 2 by clamping force happens to be too great, the clamped marginal portion 2a of the diaphragm 2 may be stretched beyond limit and torn thereat. On the other hand, too small thickness reduction at the clamped marginal portion 2a of the diaphragm 2 may result in inadequate airtight sealing thereat.
FIG. 2a shows another diaphragm device forming part of a vacuum advance mechanism of a distributor for an internal combustion engine. The structure and operation of the device of FIG. 2a is similar to those of the device of FIG. 1a, wherein like reference numerals represent like or corresponding parts or portions. However, the inner marginal portion of an annular diaphragm 2 is mounted, together with inner marginal portions of the annular stiffening disks 4a and 4b, to a top end portion 6a of a control rod 6. Namely, as shown in FIG. 2b illustrating the portion within circle B in FIG. 2a on an enlarged scale, the inner marginal portion 2b of the diaphragm 2 held between the inner marginal portions of annular stiffening disks 4a and 4b is inserted together therewith into an annular recess formed in the side surface of a cylindrical top portion 6a of a control rod 6 to be tightly clamped therein between the disks 4a and 4b, wherein the thickness of a clamped inner marginal portion 2b is reduced by the clamping pressure exerted thereon, compared with the original thickness of the unclamped portion of the diaphragm 2, so that airtight sealing thereat is ensured.
The mounting structure of the inner marginal portion 2b of the diaphragm 2 to the control rod 6 has the same disadvantage as the mounting structure of the outer marginal portion of the diaphragm shown in FIG. 1b. Namely, the unavoidable variation in the clamping thickness reduction results in a failure or an unsufficient sealing of the clamped inner marginal portion 2b of the diaphragm 2.
U.S. Pat. No. 3,572,301 discloses a vacuum advance mechanism including a diaphragm device which has a structure different from those described above. Each one of the diaphragm devices disclosed in the U.S. patent comprises two annular diaphragms, the outer marginal portions of which are held tightly between a radially extending annular flange portion of a housing member and a flat annular surface of a rigid spacer ring opposing thereto. An annular inner marginal portion of one of the diaphragms of a first device (shown in FIG. 1 thereof) is sealed by an annular inner edge portion of a carrier member which is bent over the adjoining inner marginal portion of the diaphragm and maintains it in sealing engagement with an annular gasket interposed between the bent-over inner edge of the carrier and the diaphragm. Inner marginal portions of other diaphragms are sealed by a structure similar to that shown in FIG. 2b. Thus, the mounting and sealing structures of the outer or inner marginal portions of the diaphragms disclosed in the aforementioned U.S. patent suffer the same disadvantage as the above-described mounting structures.